Hello Grant.
When I moved into my house some years back, there was an existing shed in the garden. I converted it into a runoff roof observatory. I made a roof from 16mm poly carbonate sheets on a simple wooden frame. It slides on PTFE runners. In the six years it has been in use, I have never had any condensation whatsoever on any surface. It’s well ventilated around the roofline which makes the difference I believe. A big advantage is that during the day it’s full of natural light. The whole project probably cost a few hundred quid including replacing some rotten shiplap.
Hope this helps.
Chris.